There are devices whose safeness is/are warranted by those methods.

The verb in question is attributed to "safeness". "safeness" is singular, so it requires "is". But "devices" is plural. So?

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    We don't actually use safeness in such contexts (it's safety). But either way, since the referent is a singular noun the verb form must be is. The plural verb form are has already been correctly applied to the primary subject There are devices... – FumbleFingers Reinstate Monica Jun 17 '19 at 16:43

The subject is safeness (or, rather, safety, as FumbleFingers says), so the verb needs to be singular.

There are similar cases where usage doesn't always follow this logic, but in such cases the verb usually agrees with the closest noun phrase to the verb, even if that is not the logical subject. Here the logical subject is the closest noun phrase to the verb, so that issue will not appear, and any native speaker would say "is">

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